In airplanes and trains which generate considerable noise, when information is offered by voice service to the users, problems arise with noise generated at the seats.
In use of an inside space whose boundary is formed by a continuous wall as in an airplane or a train, the place used is a kind of closed structure, and if noise sources exist inside and outside the place used, it will cause a noise environment to be fixed for the users. As a result, although it depends upon the level of noise, the noise exerts physical and mental pressure upon the user, worsening the comfortability of the place used. Particularly, in the case of offering a service to the passengers in the cabin of an airplane, worsening of the comfortability will give serious hindrance to the quality of service operation.
Particularly, in the case of an airplane, main noise sources include noise generated from machines such as propellers and engines for producing thrust forces of the airplane, and sound of air current generated as the airplane body moves in the air such as sound generated by end portions or both wings of the airplane flying through the air. In this case, inside noise makes the passengers feel uncomfortable and also causes hindrance to the voice service. Accordingly, the situation is strongly desired to be improved.
On the other hand, as a measure for reducing noise in a closed room, conventionally, a method using a passive attenuation means is commonly employed. In the conventional method, a sound insulation material having acoustic absorptivity such as partition wall material or absorptive material is arranged between the closed structure and the noise generating source. A high-density partition wall material is used as partition wall material, and a sound absorbing sheet is used as absorptive material. A material having acoustic absorptivity is generally high-density, and such a high-density material increases in weight. As the weight increases, it causes the flying fuel to increase, and the flying distance to shorten. Accordingly, it results in worsening of the economy and function as an airplane. Also, as a structural material, lowering of the strength such as being easy to be damaged and worsening of the design function such as quality of the touch cannot be ignored.
In order to solve the problem involved in the measure for noise by using a passive attenuation means, as a method of reducing noise by using an active attenuation means, a conventional method of generating sound wave having a phase opposite to the phase of noise is generally executed. By using this method, it is possible to lower the noise level at the noise source or in the vicinity thereof in order to prevent the noise from being propagated to regions required to be reduced in noise. As a specific example, an audio-erasing device has been proposed, comprising a microphone for detecting sound generated from noise sources, a controller which amplifies the electric signal input from the microphone and reverses the phase, and a speaker which converts the electric signal input from the controller to sound and transmits the sound.
Also, recently examined is a method of executing measures for noise from the viewpoint of improving the comfortability at passenger seats, which is based on a method of reducing noise by using the above-mentioned active attenuation means. For example, those proposed include a method of disposing a noise reducing device in each seat and installing a speaker, microphone and controller in the vicinity of the seat, and a method of increasing the noise reducing effect for the user by disposing a plurality of speakers and microphones in the vicinity of the user at the seat. In this case, it is necessary to take into account the positions of the speaker and microphone arranged with respect to the hearing conditions such as the user's posture, and the effect of noise reduction greatly depend upon the positions. Particularly, the speaker emitting the control sound for reducing noise must be disposed in a position close to the user's ear, and conventional method proposed is a method of correcting the positional relation between the user's ear and the speaker by changing the tilt angle and position of the speaker (e.g. refer to Patent documents 1, 2). Regarding the speaker arrangement to the user, adjustments of the acoustic device are studied in various ways, and for example, a method of correcting the positional relation between the user and the speaker by shifting the speaker position is disclosed (e.g. refer to Patent document 3).
However, when the device is installed in a seat of an airplane or the like, it is required to reduce the size and weight of the seat and also to improve the reclining function for enhancing the using convenience, resulting in considerable change in posture of the user in the seat, and it makes the environment very severe for the installation of the noise reduction device at the seat. Particularly, in the method conventionally disclosed with respect to the arrangement of speakers at the seat, the regions where speakers can be arranged are limited, and therefore, a speaker held in a large cabinet cannot be installed. Accordingly, the speaker capacity is limited, and there arises a problem of sound quality such as frequency characteristics. Also, the function is not enough to correct the speaker position with respect to posture change in the seat because of the difference in user's height and the reclining function, and there is a problem that it is difficult to realize effective reduction of noise at the user.    Patent document 1 Unexamined Japanese Patent Publication H10-143166    Patent document 2 Unexamined Japanese Utility Model Publication H5-50495    Patent document 3 Unexamined Japanese Patent Publication 2006-166281